Suspending solids in liquids in stable fashion is often problematic. Especially when the solids differ from the liquid in terms of density, they tend to sediment or to float.
One method for suspending relative large particles, for example visible capsules, is based on structured surfactant systems. The term “structured surfactant system” refers to aqueous systems that comprise surfactant structures which are larger than usual spherical micelles and whose interaction imparts thixotropic properties to the aqueous medium. These structures can be solid, can form a mesophase, or can be liquid, and can be present in the form of multi-layer spherulites, rods, disks, or flakes, which are dispersed or emulsified discontinuously in the system or form weak network structures.
Three principal types of suspended systems have been utilized in practice, all involving an Lα phase in which double layers of surfactants are arranged with the hydrophobic part of the molecule on the inner side and the hydrophilic part on the outer side of the double layer (or vice versa). The double layers are located side by side, for example in a parallel or concentric arrangement, sometimes separated by aqueous layers. Lα phases can usually be identified by their characteristic texture under a polarization microscope, and/or by X-ray diffraction.
Most surfactants form an Lα phase, at either ambient or slightly higher temperature, when they are mixed with water at certain specific ratios. Lα phases of this kind generally cannot, however, be used as structured suspended systems. Useful quantities of solids cause the systems to no longer be pourable, and smaller quantities of solids tend to sediment. The concentrations at which Lα phases occur are also often appreciably higher than the concentrations usual and/or desired in liquid washing and cleaning agents.
The principal types of structured system that are used in practice are based on dispersed lamellar, spherulitic, and attenuated lamellar phases.
Dispersed lamellar phases are two-phase systems in which the surfactant double layers are arranged as parallel plates in order to form regions made up of Lα phases that are penetrated by an aqueous phase in order to form an opaque, gel-like system.
Spherulitic phases comprise spherical bodies, usually referred to technically as “spherulites,” in which surfactant double layers are arranged as concentric shells. The spherulites usually have a diameter in the range from 0.1 to 15 μm and are dispersed in an aqueous phase in the manner of a classic emulsion. The spherulites interact to form a structured system.
Many structured surfactant systems fall between lamellar dispersed and spherulitic. The surfactant systems comprise both structure types. Systems having a more strongly spherulitic character are usually preferred, since they result in lower viscosities.
A third type of structured surfactant system comprises an expanded Lα phase. It differs from the other two types of structured system in that it is substantially a single phase, and from conventional Lα phases in that it has a broader d-spacing.
Structured surfactant systems having dispersed lamellar or spherulitic phases are typically formed by the interaction of surfactants with dissolved electrolyte salts or bases. WO 2007/08510 A1, for example, discloses structured hand dishwashing or scrubbing agents.
Structured surfactant systems of this kind, however, often exhibit no yield points or insufficiently high yield points and/or contain very large amounts of electrolyte. The latter, in washing or cleaning agents that are used in automatic washing machines or dishwashers, results in increased corrosion in the machines.
An object of the invention is therefore to provide a structured liquid washing or cleaning agent having a yield point, which is also suitable in particular for use in automatic washing machines or dishwashers.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.